1. Field of the Invention
The invention is a video jet printing process employing a unique ink composition.
2. Description of the Prior Art
Various printing processes are extensively used in commerce both for communication purposes and for marking goods. With increased sophistication and speed in manufacturing various articles, more sophisticated, flexible and rapid printing processes are needed. In addition, advances in computer technology and speed have increased the need for higher-speed printing processes to handle large data output at computer terminals.
One of the problems involved in the adoption of more sophisticated and rapid printing processes is the development of inks which are compatible with the printing process and have sufficient contrast with the surface being printed to be useful. Indeed, even adherence to surfaces is a major problem, particularly where various plastics and polymers are involved. A typical example is marking cable sheathing made of carbon-filled polyethylene.
The video jet printing process is an advantageous procedure for marking and printing on certain articles. This procedure exhibits high speeds, is automatic and does not require contact with the surface being marked. It has no moving parts and requires a minimum of printed attention. Printing may be carried out on irregular surfaces with good results.
In summary, the video jet printing process is well suited to many printing applications including, for example, marking sheathing on wire cable.
An understanding of the invention is facilitated by a description of the video jet printing system. An ink stream is developed under pressure at a nozzle assembly. This stream is modulated at ultrasonic frequencies so as to produce droplets of more or less uniform size. The droplets are directed toward the substrate to be printed. The electric charge is transferred to some of the drops in a predetermined way so that they are deflected appropriately during subsequent passage between high voltage plates. This deflection controls the vertical axis landing pattern since the deflection is proportional to the charge on ink drops. Horizontal axis spacing is controlled by frequency of strokes and the speed of the substrate. Uncharged drops are returned to the ink reservoir.
Inks usually consist of dyes dissolved in a suitable solvent such as methanol. Such an ink system is satisfactory where the background is light in color or transparent, but for dark backgrounds the contrast is insufficient. In addition, other restrictions of the characteristics of the ink system are imposed by the nature of the video jet printing system and the nature of the surface being printed. For example, each droplet of the ink produced by the print system should be approximately the same weight to be compatible with the deflection system. Some mechanism should be provided so the ink will adhere to the surface being printed. The color should be light (white or yellow) so as to produce contrast with the dark background. The viscosity should be low (preferably less than 10 cps) so as to be compatible with the printing system. The ink should be soluble in polar solvents like methanol.